Wide binaries without viable bound Newtonian orbits

TL;DR

This study investigates 26 wide binary star systems using high-resolution spectra and Gaia data, finding that some systems cannot be explained by Newtonian gravity, possibly due to dynamical disruptions or non-bound status.

Contribution

The paper provides a detailed analysis of wide binaries, identifying systems that challenge Newtonian gravity and emphasizing the importance of careful system validation in such tests.

Findings

Nine systems fit bound Newtonian orbits.

Three systems cannot be explained by Newtonian bound orbits.

Some wide binaries may be disrupted or not gravitationally bound.

Abstract

Context. Wide binaries offer a unique opportunity to test gravity in the low acceleration regime, where deviations from Newtonian dynamics may appear. Aims. We use high-resolution VLT-ESPRESSO archival spectra to study 26 wide binaries with projected separations larger than 13,000 AU. By combining precise radial velocities with Gaia proper motions and parallaxes, we aim to test whether these systems are consistent with Newtonian gravity in the low acceleration regime. Methods. We use multiple radial velocity measurements and stellar parameters to remove systems affected by unresolved triples, chance alignments, or young systems. For the remaining binaries, we combine radial velocities (corrected for convective shift and gravitational redshift) with Gaia proper motions, parallaxes, and positions to attempt bound Newtonian orbital solutions Results. Fourteen of the 26 initial systems were discarded: 12 due to radial velocity variability indicating unresolved close binaries, one hosting a faint Gaia companion, and one too young. Of the remaining 12, nine can be fitted with bound orbital solution, while three show velocity differences too large to be reconciled with any bound Newtonian orbit. Conclusions. For the three systems that cannot be fitted with a bound orbit, repeated radial velocity observations allow us to confidently exclude, with one possible exception, unresolved triple stellar companions or massive close-in planets as causes. Given their likely large 3-dimensional separations, these binaries may have been dynamically perturbed or disrupted by stellar encounters or Galactic tides, and may no longer be gravitationally bound. This highlights how utmost caution must be applied when studying wide binaries as isolated systems.